by NOVA
Oct 03, 2026
A motion sensor how does it work question is a change-detection question: the device reports a shift in infrared or reflected energy in its field of view, then a switch, hub, or app acts. It does not name who moved.
This page stays on that principle and on occupancy versus vacancy as control modes. For a full sensor class map, use the types of smart home sensors guide once; the upward category is Tuya smart home products.
A motion sensor reports a change in its field, then a later control or alert action runs.
The quote object is a motion sensor / motion detector. Encyclopedia language is blunt: it is an electrical front end that notices nearby motion and then performs a task or raises an alert.
A PIR motion sensor is the common indoor class. Microwave / ultrasonic units emit energy and watch the return. Dual-technology housings often require both channels to trip before the output fires.
One PIR reports motion in its field of view. It does not identify a person and it does not print a desk coordinate from a single quad element.
Channels that write “motion” on a line item still have not said whether the SKU switches a lighting load or only posts a radio bit. That split is the next H2 after the physics.
PIR is passive: it does not emit infrared to detect a person.
Common PIR motion detectors convert a change in incoming infrared into a voltage change. The element is PIR / pyroelectric: crystals with electrodes that shift charge when incident far-infrared changes their temperature.
Idle, both halves of a two-slot detector see similar background IR from walls and floor. A warm body crossing one half first makes a positive differential, then a negative pulse when it leaves.
Optics do the real shaping. A Fresnel lens / detection zones array chops the view into interleaved beams so walking across stripes yields a rapid change the electronics call motion. A slow, field-wide temperature drift is what the differential pair is meant to ignore.
That is why a still heat source in one zone is a weak story, and a person cutting across zones is a strong story. Sensitivity is a lens-and-threshold problem, not a camera problem.
Occupancy mode turns lighting on from detection and off after a timeout with no movement.
Vacancy mode waits for a manual on, then auto-off. Those two labels are lighting control modes sitting on top of a detector. They are not a second sensing physics.
A corridor that must light the first footstep wants occupancy mode. A daylit meeting room that should stay dark until someone hits the paddle wants vacancy mode, because the occupant can leave the lights off when windows already work.
PIR occupancy is still motion-based. Building-automation notes are explicit: if nobody moves, a PIR occupancy detector can look vacant even when a person is in the chair. Lighting guides add that PIR occupancy sensors are weaker on small desk motion than on someone walking through a door.
Presence / mmWave radar is a different job when the SKU actually prints still-person ranging. Do not paste that job onto a PIR-only wall switch.
Hold timer language from the field is useful here. Many automation stacks keep an occupancy bit true until a timeout expires without a new motion event.
From the field: HomeKit users describe occupancy as a motion sensor with an adjustable timer: new motion resets the timer; no motion through the timeout reports vacant. Source: https://www.reddit.com/r/HomeKit/comments/11hi43a/occupancy_vs_motion_sensors/
That timer is software latching. It does not mean the PIR suddenly sees a motionless body.
Name whether the SKU switches a load or only reports over radio.
A wall occupancy sensor with an integral relay can open and close the lighting circuit after the PIR pulse. Component PIR outputs cannot drive an AC load by themselves; they need a relay or a later controller.
A battery radio puck sends the same pulse as a hub / gateway / app event. The light only moves if a rule, a bound switch, or a scene is specified on the same sheet.
Photocell / illuminance is the daylight inhibit path: some lighting sensors refuse auto-on when ambient light is already high. A radio SKU may report lux without switching anything.
Zigbee 3.0 sensors need a documented Zigbee-capable hub. Wi-Fi endpoints do not inherit that hub line unless the product page says so.
Switch / relay / load pairing is the line that stops a channel from shipping a AAA radio sensor against a drawing that expected a mains occupancy switch.
False trips are thermal or electrical change in the PIR band, not random failure.
Sunlight slamming the lens, a cloud shadow on a sun-warmed object, or HVAC air blowing on the plastic window can change the window temperature fast enough to look like motion. Community threads repeat the same HVAC-vent and outdoor-sun pattern.
Ordinary glass is a different trap. Panasonic-class PIR optics watch wavelengths longer than 5 µm, which ordinary glass does not pass for a person on the other side. A pedestrian outside a window is often invisible, while headlights or sun on that same glass can still overload the element.
Encyclopedia PIR notes put typical indoor units around 10 m as a common range class, not as a MOES SKU number. Line of sight still matters: PIR does not see around masonry the way ultrasonic energy can bounce.
Do not turn this H2 into mounting heights or aim checklists. Those belong on a project-input sheet, not on this principle page.
Coverage, mount class, protocol, and load/control pairing are the four fields that make the line item quotable.
| Field | What to write | Why it changes the SKU |
|---|---|---|
| Coverage | Room job: transit, occupied-still, or threshold | PIR transit vs still-presence radar |
| Mount | Wall puck, ceiling can, or wall-switch yoke | Power class and FOV geometry follow the housing |
| Protocol | Wi-Fi endpoint vs Zigbee plus hub | Hub line item appears or disappears |
| Load pairing | Switches the fixture vs radio event only | Occupancy wall switch vs app sensor |
A second scan table keeps occupancy labels honest:
| Label on the RFQ | What the electronics actually do | Typical miss |
|---|---|---|
| PIR motion | Differential IR pulse across Fresnel zones | Expecting a seated person to hold the bit |
| Occupancy sensor (mode) | Auto-on / auto-off from detection plus timeout | Treating the name as still-presence |
| Vacancy sensor (mode) | Manual-on / auto-off | Using it in a dark stair where auto-on is required |
| Hybrid presence | Printed radar plus PIR, if the PDP says so | Copying radar ranges onto a PIR-only switch |
If those four fields are blank, a catalog number is a guess. Fill them, then open a matching live page.
After the sheet wants still-person sensing on Zigbee, a live matching radio example is the MOES ZigBee human presence PIR sensor.
Model ZSS-HP05-A prints a 24 GHz radar combined with PIR, Zigbee 3.0, dual AAA cells, and illuminance 0 to 3000 lux. That same page prints stationary detection 0 to 600 cm and moving detection 0 to 1000 cm.
Battery life is printed as more than 6 months for PIR plus radar at DC 3.3V. Average current is printed under 65 uA, and the marketing copy states a detection angle of at least 100°.
Those numbers stay on ZSS-HP05-A. They are not a family-wide motion-sensor spec and they are not an AC occupancy switch rating.
Zigbee 3.0 on that SKU needs a hub path. The wired multi-mode gateway page prints 5V 1A power and Wi-Fi 2.4 GHz with ZigBee, BLE, and Mesh, and it names a human presence sensor among security automations.
Skip this SKU when the RFQ is a mains occupancy or vacancy wall switch, a 3-way traveler device, or a PIR-only puck with no radar line. Do not invent IP ratings or Matter on this page.
Two pyroelectric halves compare infrared. A person crossing Fresnel zones creates a differential pulse; the electronics treat that pulse as motion.
Occupancy is auto-on and auto-off from detection. Vacancy is manual-on and auto-off. Both can share the same PIR hardware.
Often only by timeout software: occupancy holds busy until no new motion through the delay. True still-presence needs a SKU that prints radar or equivalent, not the occupancy name alone.
Usually no. The far-infrared band used for people does not pass ordinary glass well. Sun or headlights on the glass can still create a false pulse.
Rapid temperature change on the lens or in the field looks like motion. Direct sun, cloud shadows, and air blowing on the window are classic sources.
Not reliably. PIR occupancy needs enough movement to recross zones before the timeout. Seated work is the usual miss.
A Zigbee 3.0 sensor such as ZSS-HP05-A needs a Zigbee-capable gateway on the same quote. Do not assume a Wi-Fi path unless that SKU prints Wi-Fi.
Coverage job, mount/power class, protocol including hub, and whether the device switches a load or only reports an event.
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